Why is the color ramp in the compositor treating my transparent pixels as black?
...and yes, shame on me, big Blender community, i did not answer the question &quot;why is the colorramp....&quot;, but hopefully it is useful for stillsleep and that's what he wanted to achieve.
If i understand you right, you want to

convert black to transparent

convert white to transparent


if yes, here is the node setup:
Original:


convert black to transparent



convert white to transparent


and just in case you wanna make both transparent, black and white, you can use this setup: